Hydrofoil Supported Sailing Cat

I'm doing my thesis on hydrofoil supported sailing catamarans and as a starting point, i'm looking for some lines drawings of typical hull shape for around a 40 foot racer cruiser. Anybody got any suggestions where I might find such drawings on the net (free to download) They don't need to be high tech, just a fair representation of a typical 40' racer cruiser.

Not free off the web, but I've found Kurt Hughes' catalog to be a good starting point. It doesn't have complete lines drawings, but it does have profile and typical sections for a wide range of designs.

John Shuttleworth's outstanding article on the http://Dogstar 50 has a great discussion of crusing cat cross section design. It, too, has profile shape and maximum cross section. Shuttleworth's article on multihull seaworthiness covers how to arrange the buoyancy as a function of heave so as to control the trim, and this has a huge influence on the hull shaping. His catalog has an article on the design tradeoffs in mulithull sections and statistical data on a large number of cruising and multihull designs.

Since minimum wetted area and not form stability is the driving influence on multihull shapes, you can pretty well determine the lines yourself by taking the product of the profile and typical section shape.

Lorsail: One of the problems with the concept of supporting a sailing boat on hydrofoils is that the boat needs to reach a certain speed before the foils become effective (like a plane needs a runway to get to the right speed before it can take off) Below this 'hump' speed (from graph of drag versus speed) the foils simply add drag due to the increased wetted area. In the case of a motor boat, speed is generally not a problem however sailing boats are relatively slow. This means that a very efficient (high aspect ratio) foil will be required and a boat which willbe able to acheive this speed. Since cats are generally a bit faster than monohulls, they are a more suitable application. Trimarans have comparable spans and the same advantage of speed, however catamarans were selecte by by supervisor. this doesn't however mean that hydrofoil supported monohulls (and tri's) are not viable, it's just that cats are the most obvious option. Another advatage is that the hydrofoil may be placed below the tunnel, between the demi-hulls, where it is more protected (less likely to become caught on anchor lines, etc) a further advantage would be that there are definite strength advantages in eing able to support a hydrfoil at (or near) it's ends, which is more feasible for multihulls.

Unfortunately, I think your instructor made a bad choice. I prefer trimarans as foilers, too. To me, the point of flying is to extend the speed range to higher speeds, to improve seaworthiness when traveling at high speed in a seaway, and to improve seaworthiness when hullborne in conditions too rough to fly. Foils allow the boat to "change gears" the way one does with the rig by changing sails. So the hull can be optimized for light wind conditions, when both the amas and foils are pretty much out of the water. A catamaran has excessive wetted area in the low speed regime compared to a trimaran.

You're absolutely right about foils adding drag at low speeds. Induced drag for a given lift (lift = weight) goes up inversely with speed squared. So as the speed drops toward zero, the drag increases to infinity. But supporting the boat with buoyancy has the drag go to zero as the speed drops. Regardless of the foil design, there must be a crossover where there's less drag floating than flying. It makes no sense at all to fly below that speed. For the same reason, the foils should be fully retractable. Although the low speed drag penalty may be acceptable - look at Rohan Veal's performance in the Moth World's under a wide range of conditions.

Similarly, in principle, you can take off at any speed - just make the foils large enough. If you go with tiny foils, the takeoff speed is high, and the boat can only reach takeoff in high winds. Conversely, if the foils are huge, the takeoff speed is low, but it still takes a lot of wind just to push all that claptrap through the water. So in between is a foil size that will allow the craft to take off with the minimal wind. That's the optimum foil size if flying as much as possible is the goal. But that's not necessarily the best performance - especially for a fully submerged foil that can't change its area.

Greg Ketterman (Longshot, Trifoiler) told me it's always better to err on the side of small foils, rather than large. The performance penalty from large foils just isn't worth their wider range of flying conditions. I think if you look at most foilers, the takeoff speed is around 10 - 12 kt.

I was thinking that by using fully submerged foils with independent wand type altitude control systems H would have the ability to generate righting moment and by using a trimaran platform the mainfoils could be widely separated allowing for less load on each main foil.
Also ,the temptation may be to have four foils on a cat which would be quite a bit more drag than the three normally required on a tri..

Before we go any futher, i just want to explain the aim of my thesis a bit better. For the most part, hydrofoil supported sailing boats are fully supported and buoyancy doesn't play a part. My supervisors experience is with the development of the HYSUCAT which are power boats where a portion of the weight is supported by the foils. Here we are attempting to apply the same concept to sailing boats. South Africa has a sizable industry which produces around 40' catamarans which are designed for both luxury and reasonable speed. I'd guess that a foil system would not be able to support these boats fully and efficiently, however a marked improvement in their performance (increased speed with the additional advantage of increase stability) is expected for partial support. A further advantage of that is that any pitching or heeling may be dampened out by buoyancy forces as the hulls are now on the surface.

I'd expect downwind sailing on fully supported hydrofoil boats to be problamatic for stability as any sudden pitch forward would dip the nose of the hulls, thus moving the CLR forward rapidly, thus creating yaw instability and/or pitchpole. (a sudden gust or sailing into the back of a steep wave would do the trick)

The idea is not to produce the best hydrofoil supported boat but rather to test the viability of this application. As buoyancy is far less efficient than lift created by hydrofoils (generally) is would be ideal to have fully supported boats (from a drag perpsective) however the power to displacement ratios of many cruiser boats may not be sufficient for this.

To answer your question Doug, it's been proposed that we test fully submerged foils. The righting moment will be created by the hulls and the surface effets of foils (lift drops as foils near the surface) Although dihedral foils provide a very effective self righting moment, they are less efficient.

I have a quick question. Does anyone know what lead is used generally for cats? I found some suggested leads in principles of yacht design but none for cats. My gut tells me that it woul be pretty much balanced.

H, partial foiling or the idea of reducing displacement by using foils has/is being tried on C-class cats("banana boards") and proposed for A class Cats. Open 60 tri's use a similar concept with the curved foils on their amas. But the problem with any foil that has no altitude control system is that it may lift more than you want it to. That has caused crashes in the Open 60's and is a princible reason why Nigel Irens didn't use them on Ellens boat.
I think partial lift is a viable concept but I don't agree with your dismissal of fully submerged foils on tris in waves. If the boat is in condiions where the foils are not productive they should be retracted but othewise full flying has a lot to offer and ,with the exception of hitting objects, should provide a fast, safe ride. Unfortunately, from what I read and hear the ocean seems to be full of flight stopping trash these days.
I'll stick with my small foilers but wish you luck on your project....

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